A photodiode structure of the mentioned type is known from U.S. Pat. No. 4,107,722. This known photodiode structure comprises a silicon substrate of p-type conductivity, a first layer of n-type conductivity having a relatively low doping density which is arranged on the substrate, a second layer of n-type conductivity arranged on the first layer and having a higher doping density than the first layer, and a layer of silicon dioxide arranged on the second layer. The photodiode structure is used for image detectors and scanners and is characterized by an enhanced blue color light response.
Conventional photodiode structures are prone to some deficiencies which will subsequently be explained with reference to FIG. 1. FIG. 1 illustrates the general principle of photo conductive detection with silicon diodes. The photodiode comprises an insulating layer 8, a first semiconductor layer 3 of a first conductivity type and a second semiconductor layer 4 of a second conductivity type. The photons 5 are absorbed in the silicon generating a charge pair 6. In impurity doped devices, only the minority carriers in the given layer need to be considered for the model. The generated charge is detected (via connectors 1a and 1b) as the minority carrier is able to diffuse to and cross the junction of the diode. A competitive process is the recombination of the minority carrier at defect centers 7. This process reduces the quantum efficiency. The defect center concentration is very high at the surface and is usually increasing with high doping and bad lattice match of the dopant.
The absorption is very low in the infrared spectral region. The minority carriers are generated deeply in the silicon. The diffusion length to the junction is long and the limited lifetime due to active defect centers reduces the quantum efficiency. In photodiode arrays, a long lifetime and diffusion length leads to cross talk between adjacent diodes.
In contrast to the infrared spectral region, the absorption in the ultraviolet (UV) region is very high. The minority carriers are generated very close to the surface of the device. The high photon energy generates defects and traps near or at the surface of the silicon. The quantum efficiency is reduced on the long run. This problem is known as photodegradation. Upon a sudden exposure to light, the generated traps are slowly filled with charge. Therefore, the electrical field near the surface changes with exposure. This results in changing quantum efficiency. The consequence is a drifting detector signal, also referred to as "transients".